Coal is probably the most plentiful hydrocarbon fuel on earth. However, a large volume of the coal deposits are in deep underground formations. There have been a number of efforts to exploit this energy source. Wells have been drilled into these deep coal formations and the hydrocarbon gases present in the formations have been extracted from the wells. These coal formations have been fractured utilizing techniques borrowed from the oil and gas well technology, using fluids such as water, gels, or foam fractionation, along with sand proppant, to create fractures in the formation to increase the production of gas from the coal formation. Gas produced by coal formations is relatively clean and may be processed in a manner similar to natural gas and transported via pipelines.
Another technique to exploit the energy reserve in deep coal beds has been to drill two or more wells into the coal formation. The coal in the formation is set on fire at one of the wells and the gaseous products of the burning coal are extracted through other wells. This method produces a gas product which was relatively dirty containing carbon monoxide, long-chain hydrocarbons, and other combustion products from the burning coal. Thus, the produced fuel gas is useful in powering electric generators located near the well heads, such a procedure is illustrated in U.S. Pat. No. 4,271,904 entitled “Method for Controlling Underground Combustion”. The gas produced by the burning coal is of limited value for transportation of a clean gas which is competitive with natural gas.
There has been a continuing need for a method to more effectively fracture underground coal formations to recover more of the gas present in the formation. There has been a need for an improved process to recover the gas in a relatively clean form. The present invention has the advantages of being economical, of greatly extending the fracturing of subterranean coal formations, and of increasing the recovery of clean gas without the combustion products from burning underground coal in-situ to produce a product gas.